The invention relates to a control arrangement for controlling the transmission of power to the axles and wheels of four-wheel drive vehicles.
Motor vehicles having a drive system with only two driven axles often have insufficient traction when the vehicle is accelerated under bad road conditions. For this reason, high-performance and cross-country motor vehicles are often equipped with a permanent four-wheel drive. Generally, the driving behavior of such vehicles can stand to be improved, especially in as much as maneuverability and steering in tight curves is concerned. In addition, good performance characteristics sometimes lead to an overrating of the overall driving behavior of the vehicle which can result in critical driving situations, especially when the vehicle (despite the overall good performance characteristics) can no longer be steered along an indicated course of the turn. A "catching" of this vehicle by drivers of limited experience, which in the case of conventionally driven vehicles, can be achieved by causing a measured change of load or by applying a rear-axle-locking brake, is therefore not possible.
In order to improve the traction, it is suggested in U.S. Pat. No. 3,411,601 to distribute the driving torque generated by the internal-combustion engine at the two driven axles, taking into account the loads placed on the driving axles under varying driving conditions and the maximally applicable pull forces. However, this takes place only as a function of the vehicle acceleration (or as a function of the speed or the gear), so that driving around curves is not improved. In addition, mechanical requirements are considerable because in each case two hydrodynamic torque converters and transmissions are required.
A switching-off of the locking device for differential gears of motor vehicles as a function of the steering angle is known from German Patent No. 459,638. In this type of system, the maneuverability of the vehicle is improved considerably, but an abrupt switching-off of the lock under bad road conditions can lead to unexpected vehicle driving instability resulting from the jolt caused by the inertia of the drive system.
It is therefore an object of this invention to provide an arrangement for the control of the power transmission to the axles of a four-wheel drive motor vehicle in which the traction advantages of four-wheel drive and the maneuvering advantages of two-wheel drive are combined, while avoiding the respective disadvantages of either system and while maintaining a simple mechanical structure with a relatively small number of sensed operating and driving parameters and requiring relatively low-level control technology to implement.
This and other objects are achieved in a control arrangement for a four-wheel drive vehicle which is provided with at least one differential or other gear box having adjustable power transmission characteristics. The control arrangement includes an adjustment element on the adjustable gear box for receiving a control signal and for adjusting the power transmission characteristics in response to that signal, and control circuitry for producing the control signal as a function of a calculated traction force difference at the wheels of th auxiliary (i.e., front) axle. In a preferred embodiment, the control signal is calculated by subtracting a vehicle driving resistance from the magnitude of a desired traction force. The vehicle driving resistance is calculated, in accordance with a known characteristic relationship, from the vehicle driving speed. The magnitude of the desired traction force is calculated from the vehicle driving speed and from a performance standard which is based upon knowledge of the vehicle operating characteristics and/or parameters. In another preferred embodiment of the invention, the control signal is calculated as a function of a speed difference between the input and output shafts (or between two output shafts) of the adjustable differential or gear box. In still another embodiment, the control signal is produced by control circuitry which incorporates both techniques of the first two embodiments. In this case, intermediate control values are produced and combined in a selected functional relationship to produce the control signal. In still another embodiments of the invention, the vehicle is provided with more than one adjustable gear box or differential, and with control circuitry for producing appropriate control signals to adjust the respective differentials in response to vehicle operating parameters.
The advantages of the invention are mainly the small number and good measurability of the sensed signals, the relatively low control requirements, the simple mechanical construction, and the resulting improvement in traction and driving behavior. The automatic control of the power transmission makes it possible for the drive to concentrate primarily on driving and if necessary in special driving conditions, to also intervene in the control. In addition, this arrangement reduces the wear of the tires of four-wheel drive vehicles and keeps the mechanical stress to the power transmission units, caused, for example, by distortions, within limits and ensures a good running performance of all components.